adamprohack

<h3>What does this plugin do?</h3>
This plugin generate image thumbnails only when site need it, when someone call function with needed size, 
and saves disk space when image is uploaded to media library. 
You don’t need to use <code>add_image_size()</code> function.
<h3>How it works?</h3>
<ol>
<li>Upload image to media library</li>
<li>Change function <code>wp_get_attachment_image_url()</code> to <code>get_otf_image_url()</code> in your wordpress template</li>
<li>When function is invoke the first time, the image is dynamically adjust, create and stored</li>
<li>Page serve image in need size</li>
</ol>
<h3>Documentation</h3>
<pre><code>`php
function get_otf_image_url(
 $image_id, $size_name, 
 $width = 0, $height = 0, 
 $crop_mode = ["center", "center"], 
 $mime_type = "image/jpeg", $quality = 70)
`
</code></pre>
Parameters
<ul>
<li>Required: 
*$image_id – attachment id 
***$size_name** – dynamic size name 
***$width** – image width 
***$height** – image height</li>
<li>Optional 
*$crop_mode – image crop mode 
***$mime_type** – result image mime type 
***$quality** – result image quality</li>
</ul>
Return Value
<ul>
<li>Returns an url string</li>
</ul>

WP Images OTF Plugin

mores

This plugin is basically just a little improvement of <a href="https://wordpress.org/extend/plugins/resize-at-upload/" title="Resize-At-Upload" rel="ugc">Resize-At-Upload</a>, a fabulous plugin by A. Huizinga.
The plugin will automatically resize an image upon upload, depending on the maximum width and height that you define. Gone are the days when you, or your client, will ruin a site’s layout by uploading a huge file with 25 megapixels. 
Be advised: there is no backup, no copy of the originally uploaded image.
The plugin uses a class originally from Jacob Wyke (www.redvodkajelly.com).

Resize at Upload Plus

Automatically resize (i.e. scale down) images after uploading. Save disk space and preserve your layout

WP Images OTF Plugin vs Resize at Upload Plus